This invention relates to fluidized bed combustion furnaces and more particularly to improvements in the devices provided in the horizontal dispersion plates at the bottom parts of such furnaces for injecting fluidizing air into the fluidizable beds.
Thermal apparatus utilizing the fluidized bed technique, such as coal-burning boilers, garbage incinerators, and wood-chip-burning boilers, have been and are being developed and reduced to practice.
In a typical conventional fluidized bed combustion furnace, a fluidizable substance in particulate or granular form is placed on a horizontal air dispersion plate at a bottom part of the furnace. The air dispersion plate is provided with a plurality of air injection nozzles or orifices disposed therein at equal spacing intervals for blowing air from a blast box or wind box below the air dispersion plate upward into the mass of the fluidizable substances. While the supply flow velocity of the air is low, the fluidizable substance is in a stationary or static state, but, when the air flow velocity exceeds the fluidization starting velocity of the fluidizable substance, the granules of the fluidizable substance are wafted upward and assume the state of a fluidized layer or bed which appears as though it were effervescing.
In the case where the fluidizable substance in this state of a fluidized bed is a combustible substance, continuous combustion becomes possible by causing it to ignite.
Then, when the supply of air through the air injection orifices is stopped, the fluidizable substance subsides and reassumes its original stationary state, and the fluidized bed combustion stops.
When the combustion thus stops, the fluidizable substance at a high temperature enters the air injection nozzles and drops into the wind box to cause contamination, clogging, and even heat damage in the air supply pipe connected to the wind box.
For this reason, it is the general practice at present to place a special cover or cap, made of a heat resistant steel plate and having air passage holes, over each of the air injection nozzles so as to prevent the granules of the fluidizable substance from entering into the air injection orifices.
However, even with the use of such caps, trouble such as heat damage and wear occur under the high-temperature environment within the combustion furnace. Furthermore, when the fluidized bed combustion is stopped, the pressure within the combustion chamber of the furnace is higher than that in the wind box. For this reason, even when the cap is used, the fluidizable substance at high temperatures enters through the air passage holes thereof and further enters into the air injection orifices to drop into the wind box, thereby causing contamination or heat damage to the wind box and the air supply pipe connected thereto.
Accordingly, it is necessary to supply air into the wind box so as to balance the pressure within the furnace combustion chamber and the pressure within the wind box when the fluidized bed combustion is stopped. However, since the pressure within the furnace combustion chamber decreases continuously, the pressure within the wind box must be reduced in correspondence therewith. Control of such an adjustment is extremely troublesome.